Fuel injection devices embodied as so-called common rail systems, for a reciprocating internal combustion engine with direct fuel injection, essentially comprise a nozzle part with an injection nozzle, which part has a nozzle needle that closes the injection nozzle and that is movable in the opening position via servo hydraulics upon imposition of pressure by the fuel to be injected. The requisite pilot pressure is taken from the high-pressure part of the fuel supply, that is, the common rail. Via the pressure specification in the common rail, the injection pressure can be varied quite flexibly, and via the triggering of a servo valve, and thus of the nozzle needle, the instant of injection and duration of injection can also be adjusted with great flexibility.
However, if with the known systems not only the injection quantity is to be dimensioned, by a suitable control of the opening time, but the injection rate is also to be formed, that is, the injection quantity per unit of time is to be varied during the opening time, then the stroke of the nozzle needle must be controlled. However, the hydraulic energy of the flowing fuel is set to turbulence immediately upstream of the injection port of the injection nozzle by the so-called seat throttling, which occurs especially at a relatively short needle stroke, since the free flow cross section between the nozzle needle and the nozzle needle seat, which varies as a function of the stroke, acts as a throttle. The resultant increased turbulence in the flowing fuel in the region of the injection port affects the mixture formation, so there is no xe2x80x9cgenuinexe2x80x9d rate control. In direct fuel injection, that is, injection of the fuel directly into the cylinder chamber, this is disadvantageous. As a consequence of this increase in turbulence, at small injection quantities, for instance, [injection quantities, for instance,] (sic) combustion near the nozzle of the injected fuel quantity has been found, which adversely affects the course of the combustion process.
From U.S. Pat. No. 5,526,791, German Patent Disclosure DE-A 43 41 546, and German Utility Model DE-U 297 17 649, fuel injection devices are known that each have a valve body which can be displaced into the open position by an activated actuator and allows the inflow of fuel at high pressure. If the actuator is inactivated, a restoring spring pushes the valve body back into the closing position.
The object of the invention is to create a fuel injection device for direct fuel injection that makes it possible during the applicable injection time to vary the injection quantity, or in other words to shape the injection rate.
This object is attained by a fuel injection device for a reciprocating internal combustion engine, having a nozzle part with an injection nozzle, which part has a pressure chamber in which a nozzle needle that closes the injection nozzle is guided, which needle is movable in the opening position upon imposition of pressure by the fuel to be injected, wherein the pressure chamber communicates via a connecting channel with a control part which has a valve chamber, into which the connecting channel on the one hand and a high-pressure channel, communicating with a fuel supply, on the other discharges, and in which a valve body acting as a piston system is guided, which body is kept in the closing position on a valve seat by a valve spring, and having an actuator, which is operatively connected to the valve body and which moves the valve body in the opening direction upon activation and enables the flow from the high-pressure channel into the connecting channel, and having a compensation piston, which can be acted upon via the pressure in the connecting channel in the opposite direction from the exertion of force by the actuator.
In the fuel injection device of the invention, the nozzle part is embodied such that upon pressure imposition, the nozzle needle opens the flow cross section to the nozzle openings as completely as possible; no intermediate positions are provided. The control of the volumetric flow is effected via the valve body, provided in the control part, whose stroke is variable by means of suitable triggering of the actuator. The valve body is preferably embodied as a seat valve, to assure tightness in the closed state. The actuator is expediently embodied such that in terms of its adjustment travel, it is embodied adjustingly in proportion to the adjustment energy applied. Electrical actuators which are embodied adjustingly in proportion to voltage in terms of their adjustment travel, of the kind embodied by so-called solid-state actuators, are especially suitable for this purpose. As solid-body actuators, piezoelectric actuators can be considered in particular, but also magnetostrictive actuators. Electromagnetically functioning actuators can also be used. It is advantageous to dispose a compensation piston of suitable diameter, which can be acted upon via the pressure in the connecting channel toward the nozzle part and accordingly acts counter to the force of the actuator. This produces a so-called pressure feedback, which enables good regulability of the volumetric flow flowing from the high-pressure side to the connecting channel, and thus enables good shaping of the injection rate.
It is especially expedient if in one embodiment, the valve body is provided, on an end remote from the actuator, with a compensation piston which can be acted upon via the pressure in the connecting channel.
In a feature of the invention, it is provided that the control part has a relief valve, opening toward the low-pressure side of the fuel supply, which is associated with the connecting channel and closes upon activation of the actuator. By the disposition of a relief valve of this kind, care is taken to assure that immediately upon seating of the valve body in the control part on its valve seat, the pressure in the connecting channel toward the nozzle part is rapidly decreased, so that the nozzle needle is also guided very quickly into its closing direction.
In an especially advantageous feature of the invention, it is also provided that the control part has a pressure divider, which communicates on the one hand with the high-pressure channel and on the other with the valve body with a compensation piston, forming a piston system, and which is adjustable via the actuator. Disposing a pressure divider in the control part in this way enables dynamic adjustment of whatever injection pressure is desired. Depending on the embodiment, the arrangement can be such that depending on the type of actuator used, the injection pressure can be adjusted upstream of a pressure-controlled injection nozzle, either via the adjustment travel of the actuator or via the force of the actuator.
Further characteristics and features of the invention can be learned from the claims and the ensuing description of exemplary embodiments.